Photovoltaic applications based on organic materials are known from numerous publications. More specifically a device concept in which the photovoltaic active layer comprises a mixture of an electron donating and an electron accepting material sandwiched between two electrodes is attracting much attention.
Compounds forming such a mixture for the photovoltaic active layer are manifold. Well-known electron acceptor materials are fullerenes and/or fullerene derivatives. However, other materials like, for example, cyano-substituted conjugated polymers or perylene based small molecules are also considered as electron acceptors. Well-known electron donating materials are conjugated polymers like, for example, poly(phenylene-vinylene)s, poly(fluorene)s, poly(thienylene-vinylene)s or poly(thiophene)s. However, other organic polymers as well as numerous organic materials comprising small molecules are also considered as electron donating materials.
Many material combinations are therefore studied but mixtures involving polymeric materials are attracting much attention because they are assumed to be more suitable for large area processing.
A combination of poly(thiophene) polymers with fullerene derivatives as photovoltaic active layer is well-known. Especially the ability of having regioregular poly(thiophene) incorporated in the active compound mixture can lead to a better overlap of the optical absorption characteristics of the photovoltaic active layer with the solar spectrum. An increase of the light absorption in such a photovoltaic active layer has been described to be beneficial for the performance of the final solar cell. Such an increase can be achieved by incorporating an additional treatment into the device production process, as for example shown in “Effects of Postproduction Treatment on Plastic Solar Cells”, Franz Padinger et al., Adv. Funct. Mater. 2003, 13, No. 1, January, wherein organic solar cells are treated (after the deposition of the top electrode) simultaneously with an applied external potential higher than the open circuit voltage and a temperature higher than the polythiophene glass transition temperature. Another example of an additional treatment leading to an increased absorption is described in WO 2004/025746, the treatment comprising exposing the photovoltaic active layer to a solvent vapour at room temperature or annealing the active layer at a temperature of at least 70° C. These treatments are applied after finalizing the deposition of the active layer. In “Device annealing effect in organic solar cells with blends of region-regular poly(3-hexylthiophene) and soluble fullerene”, Youngkyoo et al., Applied Physics Letters 86, 063502 (2005) an enhanced efficiency and an increase of the light absorption of bulk heterojunction solar cells is observed after annealing at 140° C.
Since the photovoltaic active layer of these devices generally has a thickness below 1 micrometer and is sandwiched in between two metallic contacts or contact layers, high accuracy on the deposition of this layer is required. Especially the need to achieve a fully continuous film between the electrodes with a well-controlled thickness and minor surface roughness is high.
A high degree of film continuity and uniformity may generally be achieved by solution processing via spin coating, whereby the compounds are dissolved in a solvent or a mixture of solvents. Several processing drawbacks can however be associated with this coating technique, such as for example high material consumption and limited substrate sizes. The application of linear casting techniques such as, for example, doctor blading or roll-casting can overcome these problems. However, these techniques still lack the possibility of direct patterning of the deposited layer. Direct patterning can for example be beneficial for integration of photovoltaic structures into larger applications or to produce monolithic modules of photovoltaic devices. Patterning of the deposited layer offers the possibility to construct several photovoltaic devices onto a single substrate to be connected with each other. Printing techniques such as, for example, inkjet printing, screen printing, gravure printing, flexographic printing or offset printing can resolve these process limitations. The above mentioned different deposition techniques have rather diverse requirements with respect to the physical properties of the solution comprising the active layer compounds. For some of them it is for example more appropriate to have a low viscosity of the solution whereas other deposition techniques require high viscosity levels.
It is not obvious with these printing techniques to deposit a photovoltaic active layer with limited thickness (e.g. 1 micrometer or less) and to achieve at the same time the required high quality of a fully continuous film with minor surface roughness. Moreover, for a good integration of the photovoltaic structures into other applications or to construct onto a single substrate several photovoltaic devices to be connected with each other, accurate line resolution and edge definition are required.
In US2005/0276910 a method is described to improve the thickness uniformity of organic layers. The drying profile of a deposited organic solution is modified by post-processing the dried film in a high temperature and/or high humidity post-processing environment. The post-processing induces a reflow of the organic material to fill in any defects and thereby creates a more uniform and flatter film profile. Alternatively, after deposition of the organic solution, and as it is drying, a treatment comprising exposure to a high temperature and/or high humidity may be performed. Patterning of the organic layers is obtained by means of photoresist lines or bank structures.